Drop-on-demand inkjet printers use printhead nozzles that each eject a single drop of ink only when activated. Thermal inkjet and piezoelectric inkjet are two common drop-on-demand inkjet technologies.
Thermal inkjet printers use heat to generate vapor bubbles, ejecting small drops of ink through nozzles and placing them precisely on a surface to form text or images. Advantages of thermal inkjet printers include small drop sizes, high printhead operating frequency, excellent system reliability and highly controlled ink drop placement. Integrated electronics mean fewer electrical connections, faster operation and higher color resolution. Originally developed for desktop printers, thermal inkjet technology is designed to be inexpensive, quiet and easy to use.
FIGS. 1-2 illustrate a known thermal inkjet 10. Inkjet 10 includes a silicon substrate 12 that supports thin-film conductor 14 and thin-film resistor 16. An opening in photoimageable polymer barrier 18 defines firing chamber 20, which is fluidly coupled with ink channel 22 for holding ink 24. Orifice plate 26 defines ink channel orifice 28. Resistor 16 is located in the center of the floor of firing chamber 20, and upon application of electricity rapidly heats a thin layer of ink 24. A tiny fraction of ink 24 is vaporized to form expanding bubble 30 that ejects drop 32 of ink onto a print medium such as paper. Refill ink 34 is drawn into firing chamber 20 automatically for subsequent drop formation and ejection. Multiple inkjets 10 generally are disposed for ejecting ink drops through multiple orifices 28 in a single orifice plate 26.
More specifically, as shown in FIGS. 3-6, resistor 16 heats ink at more than one hundred Centigrade degrees per microsecond, causing bubble nucleation shown generally at 35 in FIG. 3 in less than about 3 microseconds. Bubble 30 expands, forming drop 32 as shown in FIG. 4, at about 3-10 microseconds from start. Bubble collapse and drop break-off occur at about 10-20 microseconds from start, as shown in FIG. 5, ejecting drop 32 and drawing in fresh refill ink 34. An ink meniscus in orifice 28 settles and ink refill completes, as shown in FIG. 6, in less than about 80 microseconds from start. Inkjet 10 heats a thin film of ink about 0.1 micrometers thick to about 340 degrees Celsius. The ink does not boil; expanding vapor bubble 30 forms to expel the ink. No moving parts are used except the ink itself.
Inkjet 10 of FIGS. 1-6 is a top-ejecting inkjet, in that orifice 28 is located above resistor 16. Other inkjet configurations are known. In side-ejecting inkjet 36 illustrated schematically in FIG. 7 in partially cut-away form, for example, orifice 38 is located to the side of resistor 16 instead of above it. FIG. 8 shows another side-ejecting inkjet 40. To simplify the disclosure, certain similar elements in FIGS. 1-8 have the same reference numerals even though those elements may not be exactly identical structurally.
FIGS. 9-10 show an example of a piezoelectric inkjet 50. Inkjet 50 uses piezoelectric transducer 52, shown in an undeflected configuration in FIG. 9, to push and pull diaphragm 54 adjacent firing chamber 56. Upon application of electricity, the resulting physical displacement (FIG. 10) of transducer 52 and diaphragm 54 ejects ink drop 58 through orifice 60. Refill ink 62 is drawn through ink channel 64 for subsequent drop formation and ejection. Inkjet 50 thus mechanically moves the mass of diaphragm 54 and the ink in firing chamber 56. Mechanical manufacturing processes typically are used to create compared to thermal inkjets.